Node for setting render colour, regardless of lighting [Blender 2.78a]
When you save the final image to disk, Blender applies the nonlinear sRGB gamma correction curve. Internally Blender works with a linear color model, which in general is also true for the color picker.
If you want your final image to have certain color values you have the option to apply gamma correction to the color that you pick by adding a Gamma node with the value 2.2, which is a close approximation to the non-constant sRGB gamma value. In this case this would lead to RGB values of 128, 48, 255 in the final image.

Alternatively, instead of adding the gamma node you can enter the desired colors in hex format as Blender's color picker does compensate gamma correction in hex mode. For this particular color this would be 8033FF. The color values in the final image here would be RGB 128, 51, 255 as originally intended. But if you later decide to use colors not expressible in hex format you would have to make trade-offs there.

If it's necessary to exactly match some values then I assume you would have to calculate the RGB values yourself by using the formula mentioned in the sRGB Wikipedia article.